Issues dynamically populating a SubForm at the OnAdd.OnLoad event

I want to know how to populate a subform at the OnAdd.OnLoad event of a form. I already know the ID of the fields I want to populate it with, as they already exist in the subform and I simply want to replicate them. I am having issues with the dynamic creation of the rows in my SubForm.

For context, I am building a feature that allows the user to create multiple versions of the same record, with minor changes, and so I built a custom action that redirects the user to the onAdd of the form, with a flag that raises the "version" by one and creates a new record, so an Edit event wouldn't suffice. The thing is, I want to load the onAdd form already populated with the data from the previous version of the record, and I got it all down except for the SubForm I'm using.

Thanks in advance.